That hoary device for exhuming old crimes--the beribboned secret packet of Old Love Letters--is given a modestly engaging outing When fledgling literary researcher John Broome sidles into the Oxford household of Sir Roderick and Lady Isabella Heron, a viciously verbal octogenarian couple who hate each other even more than they hate outsiders. But John gains Isabella's trust by petting her cat and adoring her niece, and the frail harridan gives him The Letters--the love-then-hate letters from turn-of-the-century poetess Emily Witherington (who died young and mysteriously in a cycle crash) to Sir Roddy. The packet falls into the hands of the spiteful, greedy nurse-companion (soon found drowned), the cat is poisoned (a trial run for a weed-killer-curry attempt on Isabella?), and John is forced to join the smiling, homicidal Sir R. on deserted Boar's Hill. No cleverness, but this is less fluttery than most of its ilk (only a few cries of ""Good God!"") and tart enough to arouse some interest in Clarke, an established presence in Britain but unfamiliar here.